This application relates to optical Bragg gratings in optical waveguides, and more specifically, to techniques and systems associated with fabrication of such Bragg gratings.
Optical waveguides, such as optical fibers and planar dielectric waveguides formed on substrates, may be fabricated with spatial periodic grating patterns in their cores along the longitudinal direction to form Bragg gratings. Such a Bragg grating can interact with a guided optical signal to selectively reflect each spectral component xcexB(z) that satisfies the Bragg condition, xcexB(z)=2neff(z)xcex9(z), and transmits other spectral components that fail the Bragg condition, where z represents the position along the waveguide, neff(z) the effective index of refraction, and xcex9(z) the period of the grating. The grating parameter, neff(z)xcex9(z may be a constant along the waveguide to produce Bragg reflection at a Bragg wavelength or a linearly or nonlinearly chirped function of the position z to produce Bragg reflection within a Bragg reflection band.
In addition, a spatial sampling pattern with a constant sampling period or a chirped sampling period greater than the grating period may be superimposed over and modulate the grating pattern. Such a sampled grating is operable to produce multiple Bragg reflective signals at different wavelengths when the underlying grating period is a constant and multiple Bragg reflective bands centered at different wavelengths when the underlying grating period is chirped.
One way of fabricating the above Bragg gratings, for example, is to use radiation-sensitive materials to form the waveguide core and expose the core to an interference pattern produced by illumination through a phase mask. The exposure produces a spatial grating pattern through the interference of at least two diffracted beams,. e.g., the xc2x11 diffraction orders, to modulate the refractive index of the waveguide core.
The final grating patterns, however, may have irregularities or errors caused by a number of factors in the fabrication process. For example, the waveguide core such as a fiber core, may not have a homogeneous spatial distribution of the radiation-sensitive dopants. As another example, the phase mask itself may have errors or defects. Those and other factors can collectively contribute to the irregularities or errors in the final grating patterns. Therefore, it may be desirable to distinguish and identify different contributions from various factors in order to improve the fabrication precision of the Bragg gratings.
The present disclosure includes techniques to measure the feature parameters and performance of a phase pattern in a phase mask for fabricating Bragg gratings in optical waveguides. In one embodiment, the second-order diffraction of a phase mask, under proper optical illumination and detection, may be used to reproduce the spectrum of a waveguide Bragg grating formed from exposing the radiation-sensitive core to the interference pattern of two first-order diffraction beams from the phase mask. Hence, the second-order diffraction may be used as a diagnostic tool for evaluating the phase mask. In addition, other high-order diffraction beams may also be used to evaluate the phase mask.